Since they first became a reality in the mid-1980's, high-density optical and magnetooptical data storage systems have enjoyed continuing commercial success. As a consequence, there has been a technological push to improve the performance of these drives by increasing the areal density, data rates, access times and the erasability of these disk drive systems. The opportunity for further innovation in this field has lead many companies to actively develop ever more sophisticated optical and magnetooptical disk drive systems.
The basic principles of magnetooptical recording are as follows. A focused laser beam, pulsed to high power for a short time, raises the temperature of a perpendicularly magnetized medium sufficiently for an externally applied magnetic field to reverse the direction of magnetization in the heated region. When the medium returns to a lower temperature the reverse-magnetized domain persists. The orientation of these magnetized domains are sensed by the laser beam during read operations. Erasure of domains may be accomplished by the same thermal process, now aided by an oppositely directed magnetic field. Such magnetooptical recoding systems are described generally in U.S. Pat. Nos. 4,825,428; 4,730,289 and 4,814,903.
Read out of information employs the polar curve effect. Linearly polarized light, reflected from a perpendicularly magnetized medium, is rotated to the left or right, according to the direction of magnetization. By checking the direction of the plane of polarization of the reflected light, magnetization transitions can be read out by the same focussed laser beam that was used for recording the information. Reading operations have virtually no influence on the recorded information because the read laser power is relatively low and causes little temperature rise.
All optical recording systems need some sort of laser focus and tracking servo mechanism to compensate for variations in radial run out and optical path length normally associated with recording mediums. To achieve extremely high track densities, there is an additional requirement that the tracking servo information be embedded within the optical or magnetooptical disk. Generally, this servo information is preformated on the disk substrate during the fabrication process.
For example, many disks have a continuous spiral groove which is used for tracking the laser beam across the disk. Between the grooves is a "land" area wherein the data is written. A tracking signal is derived from the grooves between the tracks so that the laser beam may be made to follow the land areas. As the read spot of the laser moves off the land regions and over the grooves, an error signal is optically generated. This error signal is then fed back to the tracking servo mechanism, which acts to keep the spot centered directly over the land areas.
Focus and tracking servo mechanisms all rely on the divergence of the reflected rays increasing (decreasing) near a photo detector as the disk approaches (moves away from) the objective lens. Most often, this change in divergence is translated into a change in differential output using an ordinary operational amplifier.
Conventional focusing methods include the knife edge, biprism, obscuration, and astigmatic methods. By way of example, in the astigmatic method, a cylindrical lens is placed within the detector module. If the laser beam is out of focus on the disk, its image on a quadrant photo detector will be elliptical, with the major access orientation dependent on the out-of-focus direction. The error signal developed is fed back into the servo controller to adjust the astigmatic lens of the optic system accordingly. Together, the focusing and tracking signals maintain the laser spot in precise position relative to the disk surface and data tracks. Representative focusing and tracking error detection schemes are described in U.S. Pat. Nos. 4,764,912; 4,742,506; 4,783,590 and 4,844,617.
One problem commonly encountered in prior art optical and magnetooptical systems is focus/tracking crosstalk. Crosstalk of the tracking signal into the focus channel arises during seek operations, wherein the read laser spot is moved radially to access data information recorded in any given location on the disk. During a seek, the read spot of the laser is generally moved perpendicular to the circumferential grooves on the disk rather than tangential. This movement is along the radius of the disk. As the spot crosses a groove, the light is scattered or defracted, which results in defocussing of the laser beam. Therefore focus must be continually reacquired after the crossing of each groove. This procedure, whereby the beam is successively defocussed and refocused when crossing a groove, is one of the limiting factors contributing to long access times in prior art systems.
Past approaches to ameliorate this situation include using an external transducer to determine the velocity and/or the position of the laser spot on the disk on a continual basis. However, even with this technique the relative position of the spot on the disk is not always accurately known or predictable. Consequently, access times and reliability continue to suffer. Other attempts to reduce the tracking focus crosstalk typically rely on precise control of the system optics. This involves reducing the astigmatism and wavefront aberrations of the read spot to extremely low levels. The drawback of this technique is that the very tight tolerances, lens astigmatism and wavefront aberrations make the system difficult to align and expensive to manufacture. Often this results in very low production yields (normally less than 50%).
It will be seen later that the present invention virtually eliminates the problem of tracking/focus crosstalk in optical and magnetooptical disk drives by allowing the system to remain at optimum focus during seek operations--thus avoiding degradation in either the tracking or focus error signals. According to the present invention, the read spot on the disk remains at its optimum size and shape at all times for best resolution of the tracking error signal. In addition to greatly improving the optical quality of laser beam, the present invention also greatly simplifies the manufacturing of the optical components in the magnetooptical drive, leading to more reliable operation over temperature extremes and lifetime. Vast improvements in the of the optics system yield (approaching 100%) has brought about a substantial lowering of the cost of manufacture. Other features and advantages will become apparent from the detailed description which follows.